1. Field of the Invention
The present invention relates to a method for placing fluorescent single molecules on a surface of a substrate and, more particularly, to a method for placing fluorescent single molecules on a surface of a substrate such as a silicon wafer and a method for visualizing a structural defect of the surface of substrate.
2. Related Background Art
Recent advance in single molecule imaging methods based on detection of fluorescence has been realizing observations of position, orientation, motion, photochemical reaction, enzyme reaction, and relaxation process of excited state of individual pigment molecules at room temperature. Particularly, the success in determining orientations of individual pigment molecules by scanning near-field microscopy and scanning confocal microscopy is owing to fixation of pigment molecules. In this case, the pigment molecules are doped in a polymer. In contrast to it, use of an ordinary optical microscope provided with a television camera enables us to simultaneously observe a plurality of individual molecules without mechanical scanning.
A conventional, simple arraying method of single molecules is the spin coating method, by which the pigment molecules are uniformly dispersed by spreading them on a substrate by use of a polymer. It is noted here that a "single molecule" means not only one molecule, but also a group of plural molecules as long as groups can be counted one by one separately (which is also the case in the following description).
It is, however, very hard to uniformly disperse the pigment molecules by the spin coating method unless the pigment molecules are spread with a nonvolatile material of high viscosity such as the polymer. With intervention of the nonvolatile material such as the polymer, the material would readily take contamination in from the outside. In particular, when excitation light was of a short wavelength, for example, in the ultraviolet region, there was the problem that fluorescence from impurities taken in from the outside or fluorescence of the polymer itself in some cases, may obstruct measurement of fluorescence of single molecule at a high possibility.
Incidentally, to decrease defects and particles of the submicron or smaller order on a surface of a semiconductor wafer typified by a silicon wafer is important to realizing high integration of semiconductor memory, for example. Some methods for efficiently recognizing actual states of defects and particles on the surface of semiconductor wafer were proposed and are under practical use. For example, there are reports on the light-scattering method or on a method for combining the light-scattering method with atomic force microscope (hereinafter referred to as AFM) (for example, Fujino et al., Oyo Buturi Vol. 66, No. 7 1997, PP 723-733). The former method allows us to detect the particles and defects in sizes ranging from approximately the wavelength of light used to approximately one tenth thereof. The latter method also allows us not only to detect the particles and defects, but also to evaluate the sizes thereof.
A feature of the light-scattering method is the capability of readily detecting presence or absence of defects or particles scattered in a wide area (50 to 100 micron square) as compared with the sizes (of the submicron or less) of defects and particles. However, when the particles and defects have the sizes of below the diffraction limit of light, presence thereof can be detected, but the exact sizes thereof cannot be measured. It is difficult to distinguish the defects from adhering particles and it is not possible to determine whether a defect is convex or concave with respect to the surrounding surface. On the other hand, the AFM permits us to measure the exact sizes of particles and defects, even the sizes of below the diffraction limit of light which are impossible to measure by the light-scattering method. The AFM, however, has a drawback that a long measuring time is necessary for finding out the particles and defects which are scattered in the wide area as described above and locations of which are unknown.
Further, the combination of the light-scattering method with the AFM partly enables to make up for their drawbacks and make use of their advantages; but when the AFM taking the long measuring time is combined with the light-scattering method taking the measuring time not so long, the measuring time is after all determined by the measuring time of AFM. In addition, the AMF is not advantageous in terms of the cost of apparatus itself.
As described previously, the spin coating method had the problem that it had the high possibility that the fluorescence from the impurities taken in from the outside or the fluorescence of the polymer itself obstructed the measurement of fluorescence of single molecule.
Desires existed for measuring means for attaining information about distinction between particle and defect and, in the case of a defect, about whether it is convex or concave with respect to the surrounding surface in a simple manner and within a short measuring time equivalent to that by the light-scattering method.